1. Field of the Invention
The invention relates to an apparatus and method for use in the field of oil and gas recovery. More particularly, this invention relates to an apparatus having a first component such as a sensor and a second component such as a detectable device or material adapted to determine when a general interface region between two dissimilar fluids has passed a given point in a well.
2. Description of the Related Art
Cementing a wellbore is a common operation in the field of oil and gas recovery. Generally, once a wellbore has been drilled, a casing is inserted and cemented into the wellbore to seal off the annulus of the well and prevent the infiltration of water, among other things. A cement slurry is pumped down the casing and back up into the space or annulus between the casing and the wall of the wellbore. Once set, the cement slurry prevents fluid exchange between or among formation layers through which the wellbore passes and prevents gas from rising up the wellbore. This cementing process may be performed by circulating a cement slurry in a variety of ways.
For instance, it is generally known that a conventional circulating cementing operation may be performed as follows. First the liquid cement slurry is pumped down the inside of the casing. Once the desired amount of cement has been pumped inside the casing, a rubber wiper plug is inserted inside the casing. A non-cementacious displacement fluid, such as drilling mud, is then pumped into the casing thus forcing the rubber wiper plug toward the lower end of the casing. Concomitantly, as the displacement fluid is pumped behind it, the rubber wiper plug pushes or displaces the cement slurry beneath it all the way to the bottom of the casing string. Ultimately, the cement is forced for some distance up into the annulus area formed between the outside the casing and the wellbore. Typically, the end of the job is signaled by the wiper plug contacting a restriction inside the casing at the bottom of the string. When the plug contacts the restriction, a sudden pump pressure increase is seen at the surface. In this way, it can be determined when the cement has been displaced from the casing and fluid flow returning to the surface via the casing annulus stops.
The restriction inside the bottom of the casing that stops the plug in this conventional cement circulation procedure is usually a type of one-way valve, such as a float collar a float shoe, that precludes the cement slurry from flowing back inside the casing. The valve generally holds the cement in the annulus until the cement hardens. The plug and the valve may then be drilled out.
Further, it is known that the time the end of the cement slurry leaves the lower end of the casing (i.e. when the operation is complete) may be estimated, as the inner diameter, length, and thus the volume of the casing as well as the flow rate of the cement slurry and displacement fluids are known.
The conventional circulating cementing process may be time-consuming, and thus relatively expensive, as cement must be pumped all the way to the bottom of the casing and then back up into the annulus. Further, expensive chemical additives, such as curing retarders and cement fluid-loss control additives, are typically used, again increasing the cost. The loading of these expensive additives must be consistent through the entire cement slurry so that the entire slurry can withstand the high temperatures encountered near the bottom of the well. This again increases cost. Finally, present methods of determining when the slurry leaves the lower end of the casing generally require attention and action from the personnel located at the surface and may be inaccurate in some applications. For instance, if the plug were to encounter debris in the casing and became lodged in the casing, personnel at the surface could incorrectly conclude the cement had left the lower end of the casing and job was completed. In other applications, the plug may accidentally not be pumped into the casing. Thus, in some applications, it is known to attach a short piece of wire to the rubber wiper plug. Personnel on the surface may then monitor the wire, and once the entire wire is pulled into the wellbore, the surface personnel know the plug has entered the casing. However, this system only verifies that the plug has entered the casing, not that the plug has reached the bottom.
A more recent development is referred to as reverse circulating cementing. The reverse circulating cementing procedure is typically performed as follows. The cement slurry is pumped directly down the annulus formed between the casing and the wellbore. The cement slurry then forces the drilling fluids ahead of the cement displaced around the lower end of the casing and up through the inner diameter of the casing. Finally, the drilling mud is forced out of the casing at the surface of the well.
The reverse circulating cementing process is continued until the cement approaches the lower end of the casing and has just begun to flow upwardly into the casing. Present methods of determining when the cement reaches the lower end of the casing include the observation of the variation in pressure registered on a pressure gauge, again at the surface. A restricted orifice is known to be utilized to facilitate these measurements.
In other reverse circulation applications, various granular or spherical materials of pre-determined sizes may be introduced into the first portion of the cement. The shoe may have orifices also having pre-determined sizes smaller than that of the granular or spherical materials. The cement slurry's arrival at the shoe is thus signaled by a “plugging” of the orifices in the bottom of the casing string. Another, less exact, method of determining when the fluid interface reaches the shoe is to estimate the entire annular volume utilizing open hole caliper logs. Then, pumping at the surface may be discontinued when the calculated total volume has been pumped down the annulus.
In the reverse circulating cementing operation, cementing pressures against the formation are typically much lower than conventional cementing operations. The total cementing pressure exerted against the formation in a well is equal to the hydrostatic pressure plus the friction pressure of the fluids' movement past the formation and out of the well. Since the total area inside the casing is typically greater than the annular area of most wells, the frictional pressure generated by fluid moving in the casing and out of the well is typically less than if the fluid flowed out of the well via the annulus. Further, in the reverse circulating cementing operation, the cement travels the length of the string once, i.e. down the annulus one time, thus reducing the time of the cementing operation.
However, utilizing the reverse circulating cementing operation presents its own operational challenges. For instance, since the cement slurry is pumped directly into the annulus from the surface, no conventional wiper plug can be used to help displace or push the cement down the annulus. With no plug, there is nothing that will physically contact an obstruction to stop flow and cause a pressure increase at the surface.
Further, unlike the conventional circulating cementing process where the inner diameter of the casing is known, the inner diameter of the wellbore is not known with precision, since the hole is typically washed out (i.e. enlarged) at various locations. With the variance of the inner diameter of the wellbore, one cannot precisely calculate the volume of cement to reach the bottom of the casing, even when using open hole caliper logs.
Other methods of determining when the cement slurry has reached the lower end of the wellbore are known. For instance, it is known that the restrictor discussed above may comprise a sieve-like device having holes through which the drilling mud may pass. Ball sealers—rubber-covered nylon balls that are too large to go through those holes—are mixed into the cement at the mud/cement interface. In operation, as the mud/cement interface reaches the lower end of the casing, the ball sealers fill the holes in the sieve-like device, and changes in pressure are noticed at the surface thus signaling the end of the operation. Again, erroneous results may be produced from this system. The wellbore is typically far from pristine and typically includes various contaminants (i.e. chunks of shale or formation rock that are sloughed off of the wellbore) that can plug the holes. Once the holes are plugged, the flow of cement and drilling mud ceases, even though the cement interface has not reached the lower end of the casing. Also problematic is that fact that once any object is inserted into the casing, or annulus for that matter, its precise location of that object is no longer known with certainty. The accuracy of its whereabouts depends upon the quality and quantity of the instrumentation utilized at the surface.
From the above is can be seen that in either the conventional or reverse circulation cementing process, it is important to determine the exact point at which the cement completely fills the annulus from the bottom of the casing to the desired point in the annulus so that appropriate action may be taken. For instance, in the conventional circulation cement process, if mud continues to be pumped into the casing after the mud/cement interface reaches the lower end of the casing, mud will enter the annulus thus contaminating the cement and jeopardizing the effectiveness of the cement job.
Similarly, in the reverse circulating cementing process, if cement—or displacement fluids—continue to be pumped from the surface once the mud/cement interface reaches the lower end of the casing, excessive cement will enter the interior of the casing. Drilling or completion operations will be delayed while the excess cement inside the casing is drilled out.
Thus, a need exists for a more accurate system and method of determining the location of an interface between two fluids with respect to the wellbore. Particularly, in a cementing operation, a need exists for a more accurate apparatus and method of determining when the mud/cement interface, or the spacer/cement interface, reaches the lower end of a casing. Preferably, the apparatus and method will not rely on manual maneuvering at the surface of the well. Further, the apparatus and method should be able to be utilized with both the conventional circulating cementing operation and the reverse circulating cementing operation. Further, this apparatus preferably does not rely heavily on manual operations, nor operations performed at the surface.
Further, there is a need for an apparatus that performs the function of detecting when the mud/cement interface, or spacer/cement interface, reaches the lower end of the casing and, once the cement slurry is detected, will prevent any more fluid from being pumped. The system should be capable of operation without manual intervention from the surface.